[R-390] OT: 30V regulator?

[2002tii]

Ken wrote:

>I have recently been searching for a Voltage Regulator for a 50-volt 
>supply that I want to build. So far I have had no luck at all and 
>may have to consider using transistors to help with regulation 
>although on the circuit that I found one of the author's first 
>comments was "the regulation is not as good as with a IC regulator" 
>so I'm probably back to the drawing board on that one.
>
>Regarding the supply that I want to build...My wishlist is for it to 
>be fully regulated and stable 50V and adjustable over a small range 
>48-51V. I don't want to buy one, I want to build one but I just 
>cannot find a suitable regulator. Anyone have any ideas on where I 
>can find a regulator for this voltage OR a really good transistor 
>(Bipolar or FET) circuit?

If you want to build your own, there is no need to go fully discrete 
-- use an op-amp driving a level-translating transistor and a pass 
transistor.  You can use a resistor and zener off the unregulated B+ 
to develop a 24-or-so volt supply for the op-amp.  Another resistor 
and a voltage reference of your choice (zener or fancy reference 
chip) will give you the reference to feed to the non-inverting input 
of the op-amp.  The op-amp output feeds the base of a common-emitter 
transistor or the emitter of a common-base transistor, the collector 
of which goes to the unregulated B+ through a suitable resistor.  The 
collector of that voltage-translating transistor also goes to the 
base of the pass transistor, which can be connected as an emitter 
follower (collector to B+, emitter to the load) or as a 
common-emitter amplifier (emitter to B+, collector to the load).  A 
voltage divider from the regulated output (load connection) feeds the 
inverting input of the op-amp.  A small pot in the voltage divider 
gives you a little adjustment range.  A few resistors and clamp 
diodes here and there to avoid catastrophe on power-up and 
power-down, and you are all set.

If the supply is to produce regulated +50 volts, the voltage 
translating transistor will be NPN, regardless of whether it is 
connected common-emitter or common-base.  The pass transistor will be 
NPN if it is an emitter follower or PNP if it is used in a 
common-emitter configuration.  Note that the "inverting input" and 
"noninverting input" of the op-amp may be reversed depending on 
whether the voltage translating stage and pass stage are inverting 
(common emitter) or non-inverting (common-base, emitter follower).

I have successfully built regulators for over 1000 volts and up to 
1000 mA this way.

Op-amp application notes are full of circuits like these.

Best regards,

Don